When a user of a mobile telephone requests service, for example, places a call, the telephone transmits, as part of a service or call request, a pair of identification numbers. One of the numbers is a mobile telephone number and the other number uniquely identifies the telephone itself. A base station receiving the call request examines the two numbers. If it recognizes the numbers as a valid pair, which indicates that the identified telephone is authorized to use the mobile telephone number, the base station processes the call. The base station then charges the call to the identified telephone.
The mobile telephone number, which is assigned to the telephone by the applicable telephone company, is commonly referred to as a "mobile identification number" or "MIN." The number which uniquely identifies the telephone is an electronic serial number or "ESN," which is assigned to the telephone by the manufacturer. If a caller knows the ESN assigned to a particular telephone and the MIN for which the telephone is authorized, the caller may include these numbers in a call request originating from a different telephone, and thus, make an unauthorized call. A base station receiving the requests checks the included numbers and processes the "unauthorized" call. The base station then bills the call to the telephone identified in the call request. Accordingly, what is needed is a mechanism to prevent the alteration of information which identifies the originator of a call, or other service, request.
Each ESN is unique and assigned to a telephone by the manufacturer. The manufacturer places in the telephone a one-time programmable ESN memory which contains the assigned ESN. Accordingly, the ESN cannot be altered by the user. The MIN is assigned to the telephone by the telephone company. The MIN is stored in the telephone in a read-write memory which may be altered by a user, as necessary, to accommodate new MIN's.
When a user requires service, for example, when he desires to make a call, a request processor in the telephone formulates a call request by organizing the requisite information in the form of a series of words, retrieving the ESN and MIN from the respective memories and including these numbers in the appropriate words. The call request is then encoded for transmission and transmitted to the base station by a data transmitter. If a person desires to make an unauthorized call, that is, change the ESN-MIN combination for a particular call, the caller can easily change the stored MIN to the desired MIN. However, the caller can not change the ESN stored in the manufacturer-installed ESN memory. Accordingly, the caller might somehow instruct the request processor to insert in the call request an ESN stored in a different memory. The processor then inserts in the call request the ESN to which it is directed and formats and transmits the request as if the request contains the MIN and ESN assigned to the telephone. What is needed is a mechanism which assures that the manufacturer-assigned ESN is included in a call request.